Remove obsolete comment.

It was correct at the time of our friend Jorik writing it (checking
git blame), but the world has moved now and it is no longer a
generator.

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

synapse/federation/sender/transaction_manager.py

@@ -61,8 +61,6 @@ class TransactionManager(object):
         # all the edus in that transaction. This needs to be done since there is
         # no active span here, so if the edus were not received by the remote the
         # span would have no causality and it would be forgotten.
-        # The span_contexts is a generator so that it won't be evaluated if
-        # opentracing is disabled. (Yay speed!)
 
         span_contexts = []
         keep_destination = whitelisted_homeserver(destination)